This invention relates to the screen printing process, and more specifically to multicolor screen printing procedures involving accurately registering in series more than one color on an object supported on a pallet connected to a movable pallet support plate.
Screen printing is a process widely used today for a wide variety of products including art work, electronic circuits, instrument panels, and articles of clothing. For example, today's popular T shirts are often mass produced on screen printing machines capable of printing several thousand of such multicolored objects per hour. The basic process involves flooding a screen with ink, and then utilizing a squeegee to force the ink through previously prepared openings in the screen, the screen being in contact with the object to be printed upon during the print cycle of the machine.
In current screen printing machines objects, such as T shirts, are often affixed to movable pallets connected to pallet support plates. These plates are caused to be moved from one print head to another so as to sequentially impress the object to be printed upon with a different ink color. Obviously the aesthetic quality of the finished multicolored product will be greatly affected according to the accuracy with which each pallet is positioned in relationship to each of the print heads delivering a different color. And, of course, this pallet positioning accuracy must be accomplished in a ruggedly reliable and economical manner in order to produce a competitive product in quantity.
The problem of accurately positioning pallet support plates has been addressed in the past, as is evident from U.S. Pat. No. 3,795,189, issued Mar. 5, 1974. This patent teaches utilizing an oval rail for indexing a number of pallet support plates for registration with a number of printing heads located about the oval rail. Once a timed sequence places a plate beneath an appropriate printing head a pneumatically driven device then causes a locating bar 35 (FIG. 1) to meet and press against a notch 34 (FIG. 1) at the outside edge of the pallet support plate, thereby securing that plate under the print head immediately prior to the print cycle. Once the print cycle is complete, the locating bar is caused to be moved away from the notch in the plate, freeing the plate to be moved to the next color print head station.
Again, in U.S. Pat. No. 4,735,139 a screen printing machine similar to the one described above addresses the problem of positioning a pallet support plate 10 (FIG. 1) by utilizing a dual locator means. In this case the print head frame 18 (FIG. 1) itself locates the rear section of the plate 10 by means of a slot 22 on the edge of the print head frame which engages a cam follower 16 (FIG. 2) rotatably attached to the rear section of the plate 10. Simultaneously, a "V" shaped slot 23 (FIG. 1) on the outer edge of the plate is located and pressed by locator-roller 24 (FIG. 1) on outer locator assembly 25 (FIG. 1). Provisions are described for the release of both inner and outer edges of the plate at the conclusion of a print cycle in order to free the plate for movement to the next color print station.
An additional disclosure is added to U.S. Pat. No. 4,735,139 in U.S. Pat. No. 4,909,146, issued Mar. 20, 1990. This patent teaches an additional dual patent support plate locator system which is not attached to the print head itself. In this case two pneumatically operated roller support arms 60, 64 (FIG. 7) insert rollers 60A and 64A into locator notches 82, 83 (FIG. 7) at opposite ends of the pallet support plate 61 immediately prior to the print cycle, and a timed sequence then causes the rollers to release from the locator notches at the conclusion of the print cycle.
While the above referenced inventions provide useful devices and methods for positioning pallet support plates during screen printing cycles, they do not fully address the concerns of the instant invention. In all of the above disclosures means are required for locating and connecting with, at a minimum, the front end of the pallet support plate. In the instant invention the front section pallet support plate locator is completely eliminated, thereby significantly adding to the versatility of the over all printing machine, and to important simplifications in apparatus construction and maintenance.
Therefore a primary object of the invention is to accurately locate and stabilize a pallet support plate beneath a print head and the print head's associated screen during the print cycle of a screen printing machine.
An additional object of the invention is to accurately locate and stabilize a pallet support plate both front to back and laterally in the plate's spacial relation to a print head and the print head's associated screen during the print cycle of a screen printing machine.
A further object of the invention is to provide accurate location and stabilization of a pallet support plate during screen printing at each printing station, while at the same time maintaining full access to the front of the plate for loading and unloading procedures.
Another object of the invention is to provide for an economical manner for locating and stabilizing a pallet support plate during screen printing.
Still another object of the invention is to provide for reduced maintenance associated with locating and stabilizing pallet support plates in screen printing machines.